In an internal combustion engine such as an engine, there is a case where the intake air is compressed and supplied using a supercharging apparatus such as a turbocharger, thereby increasing output. As one type of this supercharging apparatus, there is a turbocharger which is configured to rotate a turbine at high speed using energy of the exhaust gas and drive a centrifugal compressor by the rotational force so as to feed the compressed air into the engine. However, the supercharging pressure is decreased when the engine speed is low in the turbocharger. And sufficient increase of the output cannot be expected. On the other hand, the electric supercharging apparatus which his drive by an electric motor has the advantage that a predetermined supercharging pressure can be obtained even at low engine rotation speed.
Since a rotor of the electric motor is generally provided with convex-shaped pole on its surface, as the motor rotation speed increases, a substantial amount of windage loss occurs due to the irregular surface shape. This can cause not only the power loss of the electric motor but wind noise caused by the rotor, which can lead to the noise increase. In order to reduce this power loss and noise, design change of the rotor may be considered, but there is a limit to this in reality. To solve this issue, it is taught, for instance in Patent Document 1, to seal the periphery of the rotor of the electric motor in a reduced pressure state so as to reduce air resistance of the rotor and also achieve improved power efficiency and reduced noise.
When the motor speed rises, calorific value increases in the motor coil where the current flows, an inverter part for the motor control, or the like. Especially when the electric motor is accelerated or when a large load is applied, a large current flows and the calorific value increases. Thus, a cooling mechanism is required. There are many types of cooling mechanisms such as air-cooled or water-cooled cooling mechanism. For instance, disclosed in Patent Document 2 is an air-cooling mechanism in which, by utilizing the differential pressure between the atmospheric pressure and the compressor outlet pressure pressurized by the electric motor, a portion of the compressed air is sprayed to the vicinity of the rotor.